Three-dimensional thin-film nitinol devices

ABSTRACT

A method of manufacturing three-dimensional thin-film nitinol (NiTi) devices includes: depositing multiple layers of nitinol and sacrificial material on a substrate. A three-dimensional thin-film nitinol device may include a first layer of nitinol and a second layer of nitinol bonded to the first layer at an area masked and not covered by the sacrificial material during deposition of the second layer.

RELATED APPLICATIONS

This application is a continuation of International Patent ApplicationNo. PCT/US2014/061836, filed Oct. 22, 2014, and entitled“THREE-DIMENSIONAL THIN-FILM NITINOL DEVICES,” which claims the benefitof U.S. Provisional Application No. 61/894,826, filed Oct. 23, 2013, andentitled “SPUTTERED TiNi THIN FILM,” and U.S. Provisional ApplicationNo. 61/896,541, filed Oct. 28, 2013, and entitled “THREE-DIMENSIONALTHIN-FILM NITINOL DEVICES,” which are hereby incorporated by referencein their entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under NIH grant1-R41-NS074576-01 awarded by the National Institutes of Health (NIH).The government has certain rights in the invention.

TECHNICAL FIELD

The present disclosure generally relates to intravascular devices and,more particularly, to production, using Nitinol thin-film techniques, ofdevices used for treatment of intracranial aneurysm.

BACKGROUND

There is a relatively narrow band of temperature in which nitinol can bemechanically stressed so as to transition at least partially into themartensite phase from the austenite phase despite being above thetransformation temperature. This property of nitinol is referred to assuperelasticity and is quite advantageous in that—as the namesuggests—superelastic nitinol is very flexible compared to conventionalmetal alloys. If the stress is removed and the nitinol is above thetransformation temperature, the nitinol will revert back to theaustenite phase and assume its unstressed original shape. For example, acylindrical nitinol stent may be deformed into the superelastic state sothat it can be packaged and delivered into a blood vessel using acatheter. As the stent is released from the catheter, the stent revertsto its original cylindrical shape in the blood vessel. Nitinol is thusalso denoted as a shape memory alloy.

In one application, nitinol may be used to construct neurovascular flowdiverter nitinol stents that may be placed in blood vessels in theregion of a cerebral aneurysm. The flow diverter stent essentially takesthe shape of the blood vessel prior to the formation of the aneurysm,which is then cutoff from the blood flow. The blood within the divertedaneurysm clots, which neutralizes the aneurysm. Although such flowdiverter therapy shows great promise, its application is extremelychallenging. The affected cerebral vessels may be very small—forexample, a vessel to be stented may have a diameter of just threemillimeters such that they are very delicate and prone to rupture.Balloon expanded stents are thus too risky for neurovascularapplications. In contrast, a superelastic nitinol stent is far safer andis also biocompatible.

To choke off the aneurysm, flow diverter stents are sheathed in a flowdiverter cover. The cover has to satisfy two opposing goals. On the onehand, the cover should inhibit blood flow into the aneurysm so that itsblood pools and thereby clots. A completely sealed cover would thus bestsatisfy such a goal. On the other hand, the aneurysm may be adjacent tovarious feeder vessels that branch off from the area to be stented. Ifthese feeder vessels are choked off by the flow diverter stent cover,the patient may suffer an ischemic stroke, a potentially catastrophiccomplication. To achieve these conflicting goals, the flow divertercover may comprise a fine wire mesh made from a thin film nitinol (forexample, 50 microns or less in thickness) to allow blood to escape fromthe flow diverter stent into any feeder vessels that would otherwise beoccluded. Fine-wire-mesh thin-film flow diverter nitinol stent coverswith perforations of 100 to 300 microns in length offer great promise.The “wire” in the fine wire mesh should be quite thin (for example, 5 to20 microns in diameter) because it is the edges of the wire that assistin the flow diverting effect. But it is very challenging to form a finewire mesh thin film cylindrical nitinol stent cover.

In particular, thin film nitinol is conventionally manufactured by beingsputtered onto a suitable substrate such as silicon. The sputtering isproblematic, however, in that the resulting thin film nitinol is proneto having an undesirable crystalline structure as opposed to a desiredamorphous state. An amorphous film can be crystallized by heating toapproximately 500° C. in a process known as annealing. Such acrystalline structure is essential is to achieving theaustenite-to-martensite phase change that is the hallmark of a shapememory alloy. But conventional sputtering techniques will often form athin film having a columnar crystalline structure. The columns are onlyloosely bound with each other such that the resulting film is quitebrittle and unsuitable. Accordingly, there is a need in the art forimproved thin film nitinol manufacturing techniques that can reliablyform high-quality amorphous thin film that may be subsequentlycrystallized through annealing.

Setting aside the difficulties with regard to forming amorphous thinfilm nitinol, it is desirable that the resulting stent cover formed fromsuitable thin film nitinol be fenestrated as discussed earlier. To formopenings in the thin film, it is conventional to etch the film usingphotolithographic techniques. The resulting opening can then be expandedby stretching the etched thin film nitinol to fully open up the desiredfenestrations such that the film forms a wire mesh analogous to achain-link fence except that there is no weaving of the resulting wiremesh. The wire mesh may be relatively thin in comparison to thefenestrations. For example, the fenestrations may have a length ofapproximately 300 microns whereas the wire itself may be just 20 acrossor even thinner. The resolution of wet etching is relatively coarse suchthat if the wire mesh is etched to the desired thinness (for example, 5to 20 microns in diameter), the mesh is then prone to tearing and otherflaws. The resolution of wet etching is relatively coarse such that ifthe wire mesh is etched to the desired thinness (for example, 5-20microns in diameter), the mesh is then prone to tearing and other flaws.

The substrate upon which the nitinol is sputtered includes a releaselayer so that the etched thin film nitinol can be removed from thesubstrate. But the etched thin film nitinol is essentially twodimensional (if one ignores the third dimension resulting from itsrelatively small thickness). This two-dimensional thin film must besealed onto itself in some fashion to form in a cylinder or other typeof three-dimensional structure. To seal one edge of the thin film toanother edge, it was known to use glue or stitching. But nitinol bondspoorly with glue. Similarly, stitching opposing edges together is alsoproblematic given the relatively tiny dimensions of the resulting wiremesh.

Given the difficulties with joining layers of nitinol to form athree-dimensional structure, it is also known to deposit nitinol onto acylindrical mandrel to form a cylindrical nitinol film. But suchdeposition is not amenable to mass production as the mandrel results injust one cylindrical structure. In contrast, conventional planartechniques can mass produce assorted cylindrical structuressimultaneously across a wafer substrate. In addition, deposit ontomandrel produces a solid film that must then be fenestrated upon removalfrom the mandrel. The resulting cylindrical structure is not amenable tophotolithographic etching so it is fenestrated using a laser, whichresults in relatively coarse features. Accordingly, there is a need inthe art for improved techniques for manufacturing fine wire mesh thinfilm nitinol three-dimensional structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view diagram illustrating a portion of a silicon wafersubstrate and associated structures, in accordance with an embodiment ofthe present disclosure.

FIG. 1B is a more detailed view of a portion of FIG. 1A as indicated.

FIG. 1C is a more detailed view of another portion of FIG. 1A asindicated.

FIG. 1D is a cross-sectional view of the silicon wafer substrate of FIG.1B along dashed line D prior to deposition of the nitinol layer.

FIG. 2 is a cross sectional view diagram showing a portion of a siliconwafer substrate and structures, in accordance with one embodiment.

FIG. 3 is a perspective view diagram showing an example of a structureformed on a silicon wafer substrate, in accordance with an embodiment.

FIG. 4 is a schematic block diagram illustrating a modified structureformed on a silicon wafer substrate, in accordance with an embodiment.

FIG. 5 is a flow diagram illustrating a method for forming a structureon a silicon substrate, in accordance with one or more embodiments.

FIG. 6 illustrates steps in the formation of a three-dimensional nitinolstructure using an aluminum bonding layer.

FIG. 7A illustrates expanded diamond-shaped fenestrations in a nitinolstent cover.

FIG. 7B is a close-up view of the longitudinal intersection betweenadjacent fenestrations in FIG. 7A.

Embodiments of the present disclosure and their advantages are bestunderstood by referring to the detailed description that follows. Itshould be appreciated that like reference numerals are used to identifylike elements illustrated in one or more of the figures, in which theshowings therein are for purposes of illustrating the embodiments andnot for purposes of limiting them.

DETAILED DESCRIPTION

The techniques and structures disclosed herein achieve economical,large-scale production of cylindrical nitinol film structures atreasonable cost. To provide low-cost mass production, nitinol film issputtered deposited onto a semiconductor wafer substrate. In the priorart, the resulting film was etched using photolithographic techniques toform the desired fenestrations. In contrast, the film disclosed hereinis sputtered deposited onto a substrate having dry-etched trenchesformed using deep reactive ion etching (DRIB) techniques. The substratetrenches correspond to the desired fenestrations in the resulting thinfilm nitinol deposited onto the etched substrate. Deep reactive ionetching of the substrate is quite advantageous as compared toconventional wet etching techniques to form the fenestrations. Forexample, deep reactive ion etching is considerably more precise and thusenables the formation of features with as little as one micron accuracy.In addition, the wet etching techniques left residue on the nitinol filmthat interfered with joining to another film so as to construct athree-dimensional structure such as a cylindrical stent cover. Incontrast, deep reactive ion etching of the substrate is entirelyseparate from the subsequent deposition of the nitinol film and thuscauses no contamination of the film.

After the film has been sputtered onto patterned silicon wafers, it maybe removed using a lift-off process by etching away a sacrificial layersuch as a chromium layer. Combining this lift-off process withmultiple-layer depositions of nitinol separated by layers of sacrificialmaterial enables fabrication of cylindrical stent covers, which arethree-dimensional in the sense that two layers are joined together alongtheir longitudinal edges such that the resulting joined layers may beopened up to form a cylinder.

The patterned substrate is prepared by deposition of a chrome lift-offlayer. Upon deposition of a first nitinol film onto the chrome lift-offlayer, a chrome sacrificial layer may be deposited through a mask ontothe first nitinol film. The mask covers substantially all of thepatterned area of the substrate except for the longitudinal edges alongwhich the first nitinol film is to be joined to a second film. Thesubsequent deposition of a second nitinol film then covers both thechrome sacrificial layer and the unmasked longitudinal edges of thefirst nitinol layer. The completed mesh may be removed from thesubstrate by etching of the chrome lift-off layer and the sacrificialchrome layer. A mandrel may be used to shape-set the mesh into thedesired cylindrical form by heating to annealing temperature.

A major problem is solved herein with respect to the deposition of thesecond nitinol layer. In particular, note that nitinol will promptlyform an oxidized surface layer upon exposure to the atmosphere. Thisoxidized layer is quite resistant to bonding to additional nitinollayers. To prevent formation of the oxidized layer, one could thus maskthe first nitinol layer and deposit the sacrificial chrome layer, andremove the mask and deposit the second nitinol layer while maintaining avacuum during the entire process. But such a procedure is of course verycumbersome with regard to aligning the mask and then removing it whilemaintaining a vacuum during the procedure. A particularly advantageousaluminum bonding layer is disclosed herein that obviates the need formaintaining a vacuum over all the manufacturing steps. In that regard,the first nitinol layer may be deposited (which of course is done in avacuum chamber) but the vacuum may be released while the mask isapplied. The subsequent deposition of the sacrificial chrome layer isperformed in the vacuum chamber. The mask may then be removed withoutmaintaining the vacuum and a reverse mask applied. As implied by thename, the reverse mask would be the complement of the mask used todeposit the chrome sacrificial layer. The reverse mask thus exposes thelongitudinal edges of the first nitinol layer along which it is to bondto the yet-to-be deposited second nitinol layer so that these edges maybe covered with an aluminum layer.

Upon deposition of the aluminum layer, the second nitinol layer may besputtered deposited in the vacuum chamber. The two nitinol layers arethus separated by the aluminum layer along the longitudinal edges wherethe two nitinol layers are to be joined. This aluminum layer is quiteadvantageous as the resulting structure may be heated to approximately500 to 600 degrees Celsius so that the aluminum partially melts. Asopposed to the oxidized aluminum surfaces, the molten aluminum is verychemically reactive and actively bonds to both nitinol layers. In thisfashion, the two nitinol layers are bonded together despite theformation of an oxidized layer on the first nitinol layer. The abilityto break the vacuum so as to assist in the mask alignment and othersteps greatly lowers manufacturing costs. In addition, the chemicalbonding of the aluminum layer to the two nitinol layers provides a verysecure bond. As discussed earlier, the conventional alternative was toglue or stitch the two layers together, which is quite unsatisfactoryfrom both a production viewpoint as well as with regard tobiocompatibility issues of the glue or problems caused by the stitching.

The deposition of the nitinol layers themselves is problematic. It wasconventional for the nitinol layers to be undesirably brittle from theformation of a columnar crystalline structure. Alternatively, thenitinol may be deposited so as to have an undesirable tensile strainthat can actually crack or break the substrate surface from theresulting tensile forces. Applicant has discovered that a very narrowrange of manufacturing parameters results in high-quality film. Incontrast, manufacture outside of these parameters results in excessivelybrittle material or undesirable tensile strain. With regard to theseparameters, the sputtering power, the distance between the sputteringtarget and the substrate, and Ar pressure are critical as will bediscussed further herein.

FIGS. 1A, 1B, 1C, and 1D illustrate a portion of a silicon wafersubstrate 100 in accordance with one or more embodiments. As shown inFIG. 1A, a thin film layer 101 may be deposited on, for example, asilicon substrate 100 using sputtering. Since the surface of thesubstrate is planar, the resulting thin film layer 101 is also planar.As seen in the cross-sectional view of FIG. 2, layer 101 comprises alift-off layer 115 that is initially deposited onto the surface ofsubstrate 100. A first NiTi layer 121 covers lift-off layer 115. Thisfirst NiTi layer 121 forms one-half of a resulting stent cover (notillustrated). The remaining half of the stent cover is formed by asecond NiTi layer 122 that is partially separated from first NiTi layer121 by a sacrificial layer 116. Sacrificial layer 116 forms what willeventually become the lumen of the resulting stent cover. NiTi layers121 and 122 are not joined together along the longitudinal edges of theresulting stent cover and thus along the longitudinal edges ofsacrificial layer 116. These longitudinal edges 112 are shown in FIG. 3after removal of the sacrificial layer 116 to form a stent cover 114. Alumen 125 for stent cover 114 exists in the place of the removedsacrificial layer 116.

To function as stent cover for neurological applications, stent cover114 should have fenestrations 106. Referring again to FIG. 1A, substrate100 may be configured so that thin film layer 101 includes the patternsof fenestrations 104 for each subsequently formed stent cover. Thesepatterns of fenestrations 104 may also be denoted as a fiche 104 in thatthe fenestrations 104 are in collapsed form on substrate 100. Just likea microfiche, each fiche 104 or pattern of fenestrations effectivelycodes for the resulting fenestrations when the stent cover is expandedto fully open up the fenestrations. The number of fiches 104 onsubstrate 100 thus determines the resulting number of stent covers 114that will be produced in one given production batch.

A close-up view of a fiche 104 is shown in FIG. 1B. Fenestrations 104 atthis stage are not expanded and thus are in the form of narrow columnarapertures. One column of apertures is staggered with regard to anadjacent column so that when the fenestrations 104 are later expanded,the resulting stent cover has a “chain link fence” mesh pattern. As willbe explained further herein, such a mesh pattern is quite advantageousfor a flow diverter stent cover.

FIG. 1D shows a cross-sectional view of the fiche 104 of FIG. 1B. But inFIG. 1D, thin film layer 101 has not yet been formed. To form thedesired fenestrations that make up a fiche or pattern of fenestrations,substrate 100 includes corresponding grooves 160 formed using a deepreactive ion etching process. Lands 170 support the subsequent thin filmlayer 101 that will form a wire mesh between adjacent fenestrations.Referring again to FIG. 1B, each fenestration 104 (prior to beingexpanded) may be approximately 5 to 20 microns across and approximately300 microns in length. Each land 170 may also be approximately 5 to 20microns across. Such a land width means that the resulting wire meshwill also have a width of approximately 5 to 20 microns across. The wiredepth depends upon the film layer 101 depth, which may be, for example,from 5 to 20 microns in depth. It will be appreciated, however, thatthese dimensions are just examples and may be varied in alternateembodiments.

Trenches or grooves 160 may be 50 microns deep in one embodiment.Following removal of lift-off layer 115 and sacrificial layer 116, NiTilayers 121 and 122 may be crystallized at 500° deg. C. for about 120minutes in a vacuum less than 1×10⁻⁷ Torr, which may produce, forexample, a 6 micron thick micropatterned Nitinol thin film sheet (e.g.,device component 114) that can be lifted off the silicon substrate(e.g., silicon wafer substrate 100).

In one embodiment, the DC sputtering process involves the use of a nearequiatomic NiTi alloy target under ultra-high vacuum (UHV) atmosphere(e.g., base pressure of a sputter chamber may be set below 5×10⁻⁸ Torrand argon (Ar) pressure about 1.5×10⁻³ Torr). The silicon wafer isrotated adjacent the heated NiTi target during deposition of the NiTi(for minimizing compositional variations) so as to fabricate a NiTi film(e.g., about 6 microns thick or in a range of about 2-12 microns thick)with a deposition rate of 0.1 microns per minute.

As seen in FIGS. 1A and 1C, individual web fiches 104 may spaced apartin a regular pattern (e.g., a web fiche pattern 102 of FIG. 1A) so thatthe un-fenestrated spaces in the web fiche pattern 102 between theindividual meshes 104 form areas 108 (also referred to as streets 108)resembling and analogous to streets on a map. Streets 108 may be formedduring the DRIE process of creating grooves 160 shown in FIG. 1D. Theremay be a significant difference in scale between the size of the streets108 (e.g., 1000 microns) and the widths for fenestrations 106 (e.g., 10microns). A mask 110 shown in FIG. 1C may be readily formed that takesadvantage of the difference in scale between the streets 108 and thefenestrations 106 of the individual web meshes 104. In this fashion,mask 110 may have a spatial alignment resolution of at least 50 micronsso that mask 110 covers streets 108 and the longitudinal edges (112 ofFIG. 3) of each individual fiche 104 to a depth of about 10 microns ormore. It is on these areas covered by mask 110 that first and secondNiTi layers 121 and 122 are joined. This joining occurs becausesacrificial layer 116 is deposited through mask 110. When mask 110 isremoved and second NiTi layer 122 deposited over sacrificial layer 116,NiTi layer 122 will be deposited onto NiTi layer 121 wherever NiTi layer121 was masked by mask 110 so that a bond 112 (shown in FIG. 2) may beformed between the contacting layers 121 and 122. In alternativeembodiments, an additional bonding layer may be deposited to assist inthe joining of layers 121 and 122 as will be explained further herein.

Sacrificial layer 116 may be sputter deposited onto first NiTi layer 121through mask 110. Mask 110 thus prevents the sacrificial (e.g., Cr)layer 116 from depositing on the streets 108 and on the longitudinaledges of each individual mesh 104 of web fiche pattern 102. The entireprocess of forming a three-dimensional object such as stent cover 114entails no use of chemical wet etch except a Cr etch of the finishedthree dimensional object to remove the sacrificial Cr layer 116 andlift-off layer 115. But since layers 121 and 122 are already joined bythat time, the wet etching causes no complications. In contrast, the wetetching of the prior art to form the fenestrations was performed priorto the joining of the nitinol layers and thus interfered with thisjoining through the resulting chemical contamination of the first NiTilayer. All of the process operations up to the final etch of thesacrificial Cr layers, which release the finished three dimensionalobject, may be carried out in a vacuum without exposure to atmosphere soas to ensure a strong bond 112 between the NiTi layers 121, 122 (e.g.,device components 114 of three dimensional device 124). The enhancedquality and strength of the bond 112 compared to other methods such asadhesive, laser welding, or suturing may, for example, provide extrareliability and safety for a stent cover device 124.

The final etch of the sacrificial Cr layers may produce, as seen in FIG.3, a device such as a stent cover 114 having a lumen 125 between twoNiTi layers that are joined (e.g., by bonds 112) at the edges. Thedevice shown in FIG. 3, although appearing flattened, can be seen to beequivalent topologically to a three dimensional cylinder. Lumen 125 maybe enlarged, as seen in FIG. 4 for example, by insertion of a mandrel,and the two NiTi layers (e.g., device components 114) may be shape set(e.g., by annealing) to form a cylindrical stent cover 124 having bonds112 between the two NiTi layers (e.g., device components 114). Becausethe bond between the two layers is strong (e.g., approaching thestrength of the NiTi material itself) bond 112 can have a width no widerthan the thickness of the individual layers. Hence bond 112 may notpresent a significant obstacle to insertion of stent cover 124 in acatheter for implantation.

FIG. 5 illustrates a method 500, in accordance with one or moreembodiments, for forming a three dimensional structure on a siliconsubstrate without wet etching, other than, for example, to release thestructure from the substrate. Although description of method 500 refersto production of individual web fiche mesh 104 or single devices 124, itcan be seen from FIG. 1A, for example, that many devices 124 can beproduced simultaneously using the method of FIG. 5.

At step 501, a first sacrificial layer (e.g., lift-off or release layer115 shown in FIG. 2) of Cr (or other sacrificial or barrier layers) maybe deposited on a silicon substrate (e.g., silicon wafer substrate 100)in a sputtering chamber while the substrate is held at high vacuum orunder ultra-high vacuum, using e-beam evaporation or PECVD, for example,as described above. When subsequently etched away, the lift-off layermay release the finished product such as device 114 from the substrate(e.g., silicon wafer substrate 100) and may thus be referred to as arelease layer. The lift-off layer may be 1700 to 3000 Angstroms ofsputter-deposited chromium.

Prior to the deposition of the lift-off layer, the substrate may first(e.g., before deposition) be prepared in step 501, as described above,by etching (using, for example, dry etching or DRIE) grooves or trenchesthat will correspond to fenestrations of a web fiche pattern 102 orother surface features that may correspond to structures (e.g., meshfenestrations) of a finished product such as device 114. Step 501 andsubsequent steps 502 through 506 may all be performed while thesubstrate continues to be held under a vacuum in a sputtering chamberand without removing the vacuum (or removing the substrate wafer ordevice from the vacuum chamber) until all depositions are completed,even during operations of manipulating a shadow mask, such as at steps503 and 505 of method 500.

At step 502, a first layer of NiTi (e.g., layer 121 shown in FIG. 2) maybe deposited using one or more sputtering or other techniques, examplesof which are described above. An example thickness of this first layer(as well as the second layer of NiTi) is 3 to 5 microns.

At step 503, a shadow mask (e.g., mask 110) may be placed over thesubstrate and the previously deposited layers such as the release layer115 and NiTi first layer 121. Manipulation (e.g., placing, removing) ofthe shadow mask may be performed without interrupting the maintainingunder vacuum of the substrate and previously deposited layers. Theshadow mask may protect covered (or blocked) areas from subsequentdeposition of a second Cr sacrificial layer (or other sacrificial orbarrier layers). The masked (covered) areas may include portions of thefirst NiTi layer 121 intended to form a bond 112 with the second NiTilayer 122 so that those same areas (e.g., edges of the individual webfiche mesh 104 to a width of about 10 microns) may be exposed afterdeposition of the second sacrificial layer. Thus, a mask 110 may beplaced with a spatial alignment resolution of 50 microns so that mask110 covers streets 108 and the edges of the individual web fiche mesh104 to a width in a range of about 5 microns to about 15 microns.

At step 504, a second sacrificial layer (e.g., layer 116 shown in FIG.2) of Cr (or other sacrificial or barrier layers) may be deposited onthe silicon substrate (e.g., silicon wafer substrate 100) in asputtering (or vacuum) chamber while the substrate continues to be heldat high vacuum or under ultra-high vacuum, using e-beam evaporation orPECVD, for example, as described above.

At step 505, the shadow mask 110 may be removed from the substrate andthe accumulated deposited layers. Removal of the shadow mask may beaccomplished without removing the vacuum or removing the substrate andaccumulated deposited layers from the vacuum.

At step 506, a second layer of NiTi (e.g., layer 122 shown in FIG. 2)may be deposited using one or more sputtering or other techniques,examples of which are described above. At this step, deposition ofsecond layer of NiTi 122 may result in second layer of NiTi 122 bondingto first layer of NiTi 121 at those areas left exposed by the secondsacrificial layer 116, forming, for example, bonds 112 at the edges ofindividual web fiche mesh 104.

At step 507, removal of the sacrificial layers (e.g., first sacrificialor release layer 115 and second sacrificial layer 116) may be performedusing a wet etch and may be performed after allowing the vacuum chamberto repressurize or after removing substrate 100 from the vacuum chamber.Etching the sacrificial layers may release the device components 114from the substrate and may remove interior layers such as secondsacrificial layer 116. The etch may comprise soaking silicon substratewafer 100 and the deposited layers in a solution, for example, of Cretch, and may create a lumen (e.g., lumen 125 shown in FIG. 3) wheresacrificial layers are removed between the first and second NiTi layersthat are joined at the edges. Further processing may include shapingdevice 124 including, for example, shaping device 114 into a morerounded shape, as shown in FIG. 4, by insertion of a mandrel into lumen125 shown in FIG. 3. With device 114 in the desired shape, the NiTilayers may be crystallized as discussed earlier.

It will be appreciated that bonding of one NiTi layer onto another canbe problematic in that NiTi readily forms an oxidized surface layer.This surface layer inhibits the bonding of one NiTi layer to another. Toprevent formation of this surface oxidized layer requires the first NiTilayer 121 to be maintained in a vacuum or a non-oxidizing environmentbefore second NiTi layer 122 may be bonded to it, which is cumbersomeand increases manufacturing costs. For example, mask 110 must be appliedand removed while maintaining a high vacuum. The bonding layer discussedbelow obviates the need to maintain such a vacuum across all themanufacturing steps. Referring now to FIG. 6, manufacturing costs may belowered as shown in the example manufacturing flowchart. The first threesteps are as described previously. In that regard, a lift-off layer 115is applied to substrate 100, followed by deposition of first NiTi layer121 and sacrificial layer 116. But before the second NiTi layer 122 isdeposited, an aluminum bonding layer is applied using a reverse mask(not illustrated). This reverse mask is (as implied by the name), thecomplement of mask 110 used to form sacrificial layer 116. In otherwords, the reverse mask covers sacrificial layers 116 and exposes theuncovered areas of first NiTi layer 121. Aluminum may then be sputteredthrough the reverse mask to form bonding layer 600. Since bonding layer600 is applied, first NiTi layer 121 may be exposed to the atmospherebetween the masking with mask 110 and the subsequent masking with thereverse mask. In this fashion, manufacturing costs are lowered in thatthe applications of the masks is greatly aided by performing the maskapplications outside of the vacuum chamber using, for example,conventional semiconductor pick-and-place equipment. After applicationof bonding layer 600, second NiTi layer 122 may be sputter deposited asdiscussed earlier. The wafer 100 may then be heated to approximately 500to 600 degrees prior to removal of the lift-off and sacrificial layers.Such heating partially melts the aluminum, which then becomes veryreactive despite the formation of some aluminum oxides. The moltenun-oxidized aluminum is very reactive and chemically bonds to the NiTilayers, resulting in a very secure bond, despite the formation of anoxidized NiTi surface on the first NiTi layer.

Regardless of whether an aluminum bonding layer is used, the resultingstent cover is quite advantageous over conventional wire meshapproaches. For example, a conventional wire mesh to function as a flowdiverter stent cover uses a wire of at least 30 to 40 microns indiameter. Such a relatively thick wire must weave up or under adjacentstrands to form the desired mesh. But the mesh from the techniquesdescribed herein is planar with regard to the wire intersections. Inthat regard, the columnar fenestrations may be expanded into diamondshapes having a length of approximately 300 microns and a width ofapproximately 150 microns. In contrast, the resulting wire forming thediamond-shaped fenestrations is only 5 to 20 microns in thickness. Each“corner” of the diamond-shaped fenestration is thus relatively flat suchthat a null region with regard to fluid flow is formed at each corner.This may be better appreciated with regard to FIG. 7A, which shows thediamond-shaped fenestrations that result upon expansion of the columnarfenestrations 104 shown earlier. As shown in the close-up view in FIG.7B for the adjacent longitudinal ends of two diamond-shapedfenestrations, the wire mesh forms regions 700 and 705 in theinterstices of the resulting flat wire mesh that are advantageouslyconducive to the desired clotting process so that flow diversion ofaneurysm is safely achieved. Such interstices are absent in aconventional wire mesh cover because of the weaving of the relativelycoarse wire. In contrast, the width W for the wire mesh of FIG. 7B maybe 10 microns or less.

As discussed earlier, DC sputtering of NiTi layers 121 and 122 isproblematic in that the resulting nitinol may be too brittle due to anundesirable columnar crystalline structure being formed upon deposition.Alternatively, the deposition may be amorphous but possess such tensilestrain that it buckles or even cracks the semiconductor substratesurface. To provide high-quality film and solve this prior-art issues,DC sputtering may be performed using the following parameters. Inparticular, the Ar pressure in the vacuum chamber should be 3 milli Torror less, more preferably 2 milli Tarr or less. The sputtering powershould be at least 1 kilowatt and more preferably at least 2 kilowatts.Finally, the distance between the sputtering target and thesemiconductor substrate surface should be between 2 and 3.5 inches.

Embodiments described herein illustrate but do not limit the disclosure.It should also be understood that numerous modifications and variationsare possible in accordance with the principles of the presentdisclosure. Accordingly, the scope of the disclosure is best definedonly by the following claims.

What is claimed is:
 1. A method comprising: deep reactive ion etching apattern of grooves on a surface of a semiconductor substrate, thegrooves corresponding to fenestrations in a desired three-dimensionalnitinol structure; depositing a lift-off layer on the groovedsemiconductor substrate surface; depositing a first NiTi layer over thelift-off layer; depositing a sacrificial layer to partially cover thefirst NiTi layer, the sacrificial layer corresponding to a lumen in thedesired three-dimensional nitinol structure; and depositing a secondNiTi layer over the sacrificial layer.
 2. The method of claim 1, furthercomprising: removing the lift-off layer and the sacrificial layer sothat the first and second NiTi layers are separated from thesemiconductor substrate and so that the lumen is formed in the resultingthree-dimensional nitinol structure.
 3. The method of claim 1, whereindepositing the lift-off layer comprises depositing a copper or chromiumlift-off layer.
 4. The method of claim 1, wherein depositing thesacrificial layer comprises depositing a chromium sacrificial layer. 5.The method of claim 1, wherein depositing the sacrificial layercomprises depositing the sacrificial layer through a first mask, themethod further comprising: depositing an aluminum bonding layer onto thefirst NiTi layer through a reverse mask prior to deposition of thesecond NiTi layer, the reverse mask being approximately a reverse imageof the first mask.
 6. The method of claim 5, further comprising heatingthe aluminum bonding layer so that the aluminum bonding layer bonds thefirst NiTi layer to the second NiTi layer.
 7. The method of claim 2,further comprising inserting a mandrel into the lumen of thethree-dimensional nitinol structure, and heating the three-dimensionalnitinol structure while it is on the mandrel to crystallize the firstand second nitinol layers.
 8. The method of claim 7, wherein thethree-dimensional nitinol structure is a flow diverter stent cover, themethod further comprising covering a flow diverter stent with the flowdiverter stent cover.
 9. A nitinol stent cover, comprising: a firstnitinol layer; an aluminum bonding layer; and a second nitinol layer,wherein the aluminum bonding layer is configured to bond longitudinaledges of the first nitinol layer to longitudinal edges of the secondnitinol layer.
 10. The nitinol stent cover of claim 9, wherein the firstnitinol layer and the second nitinol layer each includes an array offenestrations.
 11. The nitinol stent cover of claim 10, wherein thefenestrations are diamond-shaped.
 12. The nitinol stent cover of claim11, wherein the first and second nitinol layers form a wire mesh havinga width of 5 to 20 microns.
 13. The nitinol stent cover of claim 11,wherein each diamond-shaped fenestration has a longitudinal extent ofapproximately 300 microns and a lateral extent of approximately 150microns.
 14. The nitinol stent cover of claim 11 where the percent metalcoverage of the nitnol film is ≦15% when covering the fully-expandedstent backbone.
 15. The nitinol stent cover of claim 11 where thedensity of fenestrations is between 15 and 25 fenestrations per squaremillimeter when covering the fully-expanded stent backbone.
 16. Thenitinol stent cover of claim 9, further comprising a flow diverter stentwithin a lumen of the nitinol stent cover.
 17. The flow diverter stentof claim 16 where the percent metal coverage of the total device (i.e.thin film nitinol cover and stent backbone is ≦20%.